Variable speed wind turbine having a passive grid side rectifier with scalar power control and dependent pitch control

ABSTRACT

A variable speed wind turbine having a passive grid side rectifier using scalar power control and dependent pitch control is disclosed. The variable speed turbine may include an electrical generator to provide power for a power grid and a power conversion system coupled to the electrical generator. The power conversion system may include at least one passive grid side rectifier. The power conversion system may provide power to the electrical generator using the passive grid side rectifier. The variable speed wind turbine may also use scalar power control to provide more precise control of electrical quantities on the power grid. The variable speed wind turbine may further use dependent pitch control to improve responsiveness of the wind turbine.

FIELD OF THE INVENTION

[0001] The present invention relates generally to variable speed windturbines, and, more particularly, to a variable speed wind turbinehaving a passive grid side rectifier with scalar power control anddependent pitch control.

BACKGROUND OF THE INVENTION

[0002] A wind turbine is an energy converting device. It convertskinetic wind energy into electrical energy for utility power grids. Thistype of energy conversion typically involves using wind energy to turnwind blades for rotating a rotor of an electrical generator.Specifically, wind applied to the wind blades creates a force on therotor, causing the rotor to spin and convert the mechanical wind energyinto electrical energy. Hence, the electrical power for such a generatoris a function of the wind's power. Because wind speed fluctuates, theforce applied to the rotor can vary. Power grids, however, requireelectrical power at a constant frequency, such as 60 Hz or 50 Hz. Thus,a wind turbine must provide electrical power at a constant frequencythat is synchronized to the power grids.

[0003] One type of wind turbine that provides constant frequencyelectrical power is a fixed-speed wind turbine. This type of turbinerequires a generator shaft that rotates at a constant speed. Onedisadvantage of a generator shaft that rotates at a constant speed isthat it does not harness all of the wind's power at high speeds and mustbe disabled at low wind speeds. That is, a generator limits its energyconversion efficiency by rotating at a constant speed. Therefore, toobtain optimal energy conversion, the rotating generator speed should beproportional to the wind speed.

[0004] One type of wind turbine that keeps the rotating generator speedproportional to the wind speed is a variable speed wind turbine.Specifically, this type of turbine allows a generator to rotate atcontinuously variable speeds (as opposed to a few preselected speeds) toaccommodate for fluctuating wind speeds. By varying rotating generatorspeed, energy conversion can be optimized over a broader range of windspeeds. Prior variable speed wind turbines, however, require complicatedand expensive circuitry to perform power conversion and to control theturbine.

[0005] One prior variable speed wind turbine is described in U.S. Pat.No. 5,083,039, which describes a full power converter having a generatorside active rectifier coupled to a grid side active inverter via adirect current (DC) link. In this configuration, the active rectifierconverts variable frequency AC signals from the generator into a DCvoltage, which is placed on the DC link. The active inverter convertsthe DC voltage on the DC link into fixed frequency AC power for a powergrid. A disadvantage of such a configuration is that it requirescomplicated and expensive circuitry utilizing active switches (e.g.,insulated-gate bipolar transistors IGBTs) for the active rectifier andinverter. These types of active switches typically have higher powerloss during power conversion and cause unwanted high frequency harmonicson the power grid. Furthermore, both the active rectifier and invertermust be controlled. Moreover, active components are less reliable thanpassive components.

[0006] Another prior variable speed wind turbine is described in U.S.Pat. No. 6,137,187, which includes a doubly-fed induction generator anda back-to-back power converter. The power converter includes a generatorside converter coupled to a grid side converter via a DC link. Both thegenerator and grid side converters include active switches. The turbinedescribed in the '187 patent is a partial conversion system because onlya portion of the generator's rated power ever passes through theback-to-back converter. Moreover, unlike the power converter of the fullconversion system, power flows through the converter in oppositedirections. That is, power can flow to the rotor windings from the powergrid in order to excite the generator or power can flow from the rotorwindings to supplement the constant frequency AC power from the statorwith constant frequency AC power from the rotor.

[0007] To supply power from the power grid to the rotor windings throughthe back-to-back converter, the grid side converter acts as a rectifierand converts constant frequency AC signals into a DC voltage, which isplaced on the DC link. The generator side converter acts as an inverterto convert the DC voltage on the DC link into variable frequency ACsignals for the generator, so as to maintain constant frequency power onthe stator. To supply power from the rotor windings to the grid throughthe back-to-back converter, the generator side converter acts as arectifier and converts variable frequency AC signals into a DC voltage,which is placed on the DC link. The grid side converter then acts as aninverter to convert the DC voltage on the DC link into fixed frequencypower for the grid. A disadvantage of this type of back-to-backconverter is that it requires complicated and expensive circuitryutilizing active switches for both converters. As stated previously,using active switches can typically cause unwanted power loss duringpower conversion and unwanted high frequency harmonics on the powergrid. Furthermore, like the prior full power converter, both convertersmust be controlled, and active components are less reliable than passivecomponents.

[0008] One type of control of the generator side converter involvestransforming AC signals representing three phase generator electricalquantities into parameters with a coordinate transformation so that thegenerator can be controlled using DC values (which is known asPark-transformation). This type of control is a form of “field orientedcontrol” (FOC). A disadvantage of using FOC-type control is that usefulinformation regarding the AC signals may be lost in the transformationprocess. Specifically, FOC assumes that the AC signals of the threephases are symmetrical (that is, that they only differ in phase). Incertain instances, the AC signals are asymmetrical and useful ACinformation may be lost during the transformation from AC signals intoDC values.

[0009] Furthermore, because FOC loses information when transforming toDC values, FOC is unable to be used in a system that independentlycontrols the electrical quantities (e.g., voltage, current) of eachphase of the power grid. Theoretically, this should not pose a problembecause the electrical quantities for each phase of an ideal power gridshould not vary. In actuality, however, the electrical quantities oneach phase of the power grid may vary, causing uneven thermal stress todevelop on the generator and non-optimal power generation. Accordingly,it would be desirable to independently control these electricalquantities for each of the three phases of the power grid.

[0010] Another aspect of a wind turbine is a pitch controller. Typicalgenerators ramp up to a preselected constant speed of operation, knownas “rated speed.” When the generator is operating at, or just beforereaching, rated speed, the turbine controls the angle at which theturbine's blades face the wind, known as the “pitch angle” of theblades. By controlling the pitch angle, the turbine can maintain thegenerator at a rated speed. Pitch controllers, however, typicallyoperate at a low frequency as compared to power conversion controllers.Thus, pitch controllers are slow to react to rapid changes in speed,which are typically caused by wind gusts.

SUMMARY OF THE INVENTION

[0011] One aspect of the present invention discloses a variable speedwind turbine. For example, the variable speed turbine may include anelectrical generator to provide power for a power grid and a powerconversion system coupled to the electrical generator. The powerconversion system may include at least one passive grid side rectifierto power to the electrical generator. Another aspect of the presentinvention discloses a variable speed wind turbine that may use scalarpower control to provide more precise control of electrical quantitieson the power grid. Still another aspect of the present inventiondiscloses a variable speed wind turbine that may use dependent pitchcontrol to improve responsiveness of the wind turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated in, andconstitute a part of, this specification illustrate implementations ofthe invention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0013]FIG. 1 illustrates one implementation of a circuit diagram for avariable speed wind turbine having a passive grid side rectifierconfiguration;

[0014]FIG. 2 illustrates a flow diagram of a method to control the powerdissipating element of FIG. 1 at below and above synchronous speed;

[0015]FIG. 3 illustrates a block diagram of one implementation of ascalar power control and dependent pitch control processingconfiguration for a variable speed wind turbine;

[0016]FIG. 4 illustrates a processing flow diagram of one implementationof scalar power control, which can be used by the power controller ofFIG. 3;

[0017]FIG. 5 illustrates a flow diagram of a method for performingscalar power control using controllable oscillating signals;

[0018]FIG. 6 illustrates an internal block diagram of one implementationfor the main controller of FIG. 3;

[0019]FIG. 7 illustrates aspects of one implementation for the partialload controller of FIG. 6;

[0020]FIG. 8 illustrates aspects of one implementation for the full loadcontroller of FIG. 6; and

[0021]FIG. 9 illustrates a block diagram of one implementation for thepitch controller of FIG. 3.

DETAILED DESCRIPTION

[0022] Reference will now be made in detail to implementations of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

[0023] The variable speed wind turbine described herein provides asimplified power converter using a passive grid side rectifier, whichavoids using active switches. For example, the passive rectifier couldbe comprised of diodes. As such, the passive grid side rectifier doesnot require processor control and provides for a more reliable powerconverter. In particular, passive components are more reliable thanactive components. Furthermore, because active switches can cause powerloss during power conversion, the passive grid side rectifier canimprove power conversion efficiency for the wind turbine. In addition,using a passive grid side rectifier does not produce high frequencyharmonics and provides less expensive and complicated circuitry for apower converter in the wind turbine.

[0024] The wind turbine also provides instantaneous control of rotorcurrents of a generator to control the instantaneous power provided to apower grid (“scalar power control”). Scalar power control can beresponsive to the actual electrical characteristics for each phase of apower grid.

[0025] The wind turbine further uses dependent pitch control that isdependent on the power controller (“dependent pitch control”). Inparticular, one implementation discloses a low-speed pitch controllerthat receives signals or information from a high-speed power controller,thereby improving the responsiveness of the pitch controller.

[0026] As described in further detail below, the variable speed windturbine may be implemented with a doubly-fed wound rotor inductiongenerator to produce electrical power. The generator may operate atbelow synchronous speed and above synchronous speed.

[0027] Synchronous speed is the speed at which a rotor (mechanicalspeed) is rotating at the same speed as the magnetic fields in a stator.In the context of the wind turbine described below, synchronous speedcan be 1800 rpm. Typically, the stator frequency is fixed to the powergrid frequency. In the United States, the nominal power grid frequencyis 60 Hz, meaning that the stator frequency is 3600 rpm. For a generatorhaving four poles (or two pole pairs), the generator's synchronous speedwould be 3600 rpm/2 or 1800 rpm. In the following implementations,operation at below synchronous speed refers to a generator speed orrotor speed that is below 1800 rpm. Operation at above synchronous speedrefers to a rotor speed that is above 1800 rpm. The precise value forsynchronous speed in the context of this description depends on factorssuch as generator design (e.g., number of pole pairs) and utility gridfrequency (e.g., 50 Hz in Europe). The wind turbine described below canbe designed to operate at any desired synchronous speed.

[0028] In the implementations described herein, by controlling theactive elements of an electrical generator's rotor side converter orinverter and by controlling the pitch of the turbine blades, a desiredamount of constant frequency power may be supplied from the generator'sstator windings. At rotor speeds below synchronous speed, excitationpower can be supplied to the generator's rotor from a power grid usingthe passive grid side rectifier. At rotor speeds above synchronousspeed, power flow can be reversed due to excess power from theelectrical generator's rotor, Which requires that the excess power bedissipated in the power converter.

PASSIVE GRID SIDE RECTIFIER CONFIGURATION

[0029]FIG. 1 illustrates a circuit diagram of one implementation for thevariable speed wind turbine 100 having a passive grid side rectifierconfiguration consistent with the invention. Wind turbine 100 includesan electrical generator 110 having a stator 113 and a rotor 112connected to a generator rotor shaft 111. Although not shown, generatorrotor shaft 111 is connected to wind blades for wind turbine 100. Animplementation of this connection may be through a gear box (as shown inFIG. 3 at 302). In one embodiment, generator 110 is implemented as adoubly-fed wound rotor induction generator such that rotor 112 andstator 113 both include two-pole, 3 phase windings to generate electricpower from rotation of rotor shaft 111. Generator 110 supplies fixedfrequency AC signals (electrical power) to the power grid (“grid”) fromstator 113. Rotor 112 may receive slip and excitation power. for theoperation of generator 110 from the power conversion system (“powerconverter”). 150 through a passive grid side rectifier 154. Rotor 112may also direct excess generated power to converter 150, which candissipate the excess generated power.

[0030] Generator 110 is coupled to the power converter 150 via inductors140. Inductors 140 act as a filter to prevent large voltage changes onthe windings within generator 110. Power converter 150 is coupled topower transformer 180. Power transformer 180 may be, for example, a690V/480V power transformer with an integrated choke or separated choke“inductor.” In particular, power transformer 180 supplies 690V to thegrid and 480V to power converter 150. Power transformer 180 is coupledto a grid charge circuit including switches 145 and resistors 146 tocharge power converter 150, without significant inrush current, by powertransformer 180. This circuit is also coupled to an electromagneticcompatibility (EMC) filter 140, which filters harmonic distortion causedby power converter 150. An over-voltage protection (OVP) circuit 160 isalso coupled to generator 110. OVP circuit 160 operates to protect powerconverter 150 from damage in over-voltage conditions.

[0031] Generator 110 supplies power to the grid via stator 113. Stator113 connects to the grid via a delta (“Δ”) connector 131 A and mainconnecter 105 or via a Y connector 131B and main connector 105. The Aconnector 131A and main connector 105 can configure windings in stator113 so that they are in a Δ connection. The Y connector 131B and mainconnector 105 can configure windings in stator 113 so they are in a Yconnection. In one implementation, the same stator windings are used forthe Δ and Y connections. In this manner, a Y-connection reduces ironlosses in stator 113 and permits a wider speed range for low windspeeds. Thus, generator 110 can selectively provide electrical power tothe grid from stator 113 via Δ connector 131A and main connecter 105 orY connector 131B and main connecter 105. Furthermore, this allows windturbine 100 to reduce power loss by selectively connecting generator 110to the grid using the delta A connector 131A and main connecter 105 orthe Y connector 131B and main connecter 105. The grid operates as a3-phase 690V utility power grid at a fixed frequency such as 60 Hz. Thegrid may also operate at other voltages or fixed frequencies, such as 50Hz, or with a different number of phases.

Power Converter

[0032] Variable speed wind turbine 100 includes a converter processor170 coupled to power converter 150 to control components within turbine100, including regulating the turbine's output power flow andcontrolling components, such as power converter 150. In one embodiment,converter processor 170 controls active components in power converter150 so as to control total electrical quantities supplied to the grid.Such electrical quantities may include the total current and powersupplied to the grid. The operation of controlling power converter 150by converter processor 170 will be described in more detail below.

[0033] Power converter 150 includes an active generator side inverter(“active inverter 151”), DC link 152, power dissipating element 153, andpassive grid side rectifier 154 (“passive rectifier 154”). For purposesof illustration, power converter 150 is shown with single elements;however, any number of elements may be implemented in power converter150. For example, power converter 150 may include any number of passiverectifiers in parallel with passive rectifier 154. Such a configurationwould be particularly useful in situations where a wind turbine providesa low power mode and at least one higher power mode, where one or moreof the parallel rectifiers would be enabled in the higher powergenerator mode(s). Multiple power dissipating elements 153 may also beprovided.

[0034] Normal operation for wind turbine 100 is at below synchronousspeed such that power flow is directed from power converter 150 togenerator 110. Consequently, in most instances, the components of activeinverter 151 operate as an inverter to convert DC voltage on DC link 152into variable frequency AC signals for generator 110. Thus, in thefollowing implementations, active inverter 151 is referred to as an“inverter.” In certain instances, however, wind turbine 100 may operateat above synchronous speed such that power flow is reversed (i.e.,excess power is being generated from generator 110) and the componentsof active inverter 151 may be used as a rectifier. That is, when powerflow is reversed, active inverter 151 operates to convert excess powerbeing generated from generator 110 into a DC voltage for power converter150. This excess power can be dissipated or discharged by dissipatingelement 153, which will be explained in further detail below.

[0035] Active inverter 151 includes active components or switches in athree-phase bridge configuration. In one embodiment, the active switchesare IGBTs. These active switches may be other types of switches, suchas, for example, bipolar junction transistors or field effecttransistors.

[0036] In one embodiment, pulse width modulated (PWM) current regulationtechniques are used to selectively control the active switches in activeinverter 151 under a scalar control algorithm (“scalar controlalgorithm”), as described below. The scalar control algorithm allows forindividual and/or independent PWM control for each phase of rotor 112based on measured electrical quantities for each phase of the grid. Thescalar control algorithm can control, individually and/or independently,electrical quantities for each phase of the grid. While other methods ofcontrol could also be employed, such as torque control using fieldoriented control, such as that described in the '187 patent, FOC-typecontrol is implemented in a different way, performs different functions,and achieves poorer results than the power control method describedherein.

[0037] The operation of active inverter 151 at below and abovesynchronous speed will now be explained. At below synchronous speed,active inverter 151 acts as an inverter converting DC voltage on DC link152 into variable frequency AC signals that are supplied to generator110. At above synchronous speed, active inverter 151 acts as arectifier, converting variable frequency AC signals from generator 110to a DC voltage, which is placed on DC link 152. As will be described infurther detail below, when the DC voltage on DC link 152 exceeds athreshold, power dissipating element 153 will lower the voltage on DClink 152 by burning off excess power that is generated from generator110.

[0038] DC link 152 includes a series of capacitor elements. One or moresets of resistors can be added in some implementations to discharge thecapacitor elements and improve symmetry. In particular, the voltage dropacross each portion of the link should be substantially the same (orsubstantially symmetrical). DC link 152, however, may be implementedwith other types of voltage storage circuit configurations.

[0039] The operation of DC link 152 at below and above synchronous speedwill now be explained. At below synchronous speed, DC link 152 stores aconstant DC voltage, which can be mathematically calculated from thevoltage from power transformer 180 that is placed on the passiverectifier 154. In the case of the voltage from power transformer 180being 480V, the DC link voltage is 480V ×{square root}{square root over(2)}. At above synchronous speed, the voltage on DC link 152 mayincrease because power generated from rotor 112 charges DC link 152.

[0040] Power dissipating element 153 includes a pair of active switches(switches of this type are typically sold as pairs) having a commonconnection to a bum-off resistor and inductor connected in series. Thebum off resistor can be used to discharge excess voltage on DC link 152,thereby dissipating excess power being generated from generator 110. Theinductor can be used in some implementations to reduce current ripple inpower dissipating element 153 to protect it from damage. The upperswitch is either controlled or permanently biased into a high impedanceor “off” condition. Thus, in an alternate embodiment, only the lowerswitch may be provided. Additionally, the order of the circuitcomponents, e.g., the controlled switch, the resistor, and the inductorin the embodiment of FIG. 1, can be altered. In sum, power dissipatingelement can employ any structure to dissipate excess power on DC link152.

[0041] The operation of power dissipating element 153 at below and abovesynchronous speed will now be explained. At below synchronous speed, thelower switch is turned off such that power dissipating element 153 actsas an open circuit, which allows DC voltage from passive rectifier 154to be stored in DC link 152. At above synchronous speed, the lowerswitch can be selectively turned on to allow excess voltage on DC link152 to be discharged in the bum off resistor. In this process, excesspower from rotor 112 is being dissipated at above synchronous speed.

[0042] Passive rectifier 154 can include six power rectifier diodesconnected in a three phase bridge configuration. The operation ofpassive rectifier 154 at below and above synchronous speed will now beexplained. At below synchronous speed (a condition where the relativegrid voltage is higher than the DC link voltage), passive rectifier 154operates to convert fixed frequency AC signals from the power grid intoa DC voltage. The DC voltage from passive rectifier 154 is placed on DClink 152 to maintain the DC link voltage at a predetermined voltage.

[0043] In one embodiment, if the lower active switch in powerdissipating element 153 is turned off, power dissipating element 153acts as an open circuit and the DC voltage from passive rectifier 154passes directly to DC link 152. At above synchronous speed when power isbeing generated from generator 110, the DC link voltage will exceed thegrid voltage. The diodes of passive rectifier 154 act to preventconversion of the DC link voltage into a current. Accordingly, passiverectifier 154 does not operate to supply power to the grid. Moreover,the diodes comprising passive rectifier 154 and the power dissipatingelement 153 are designed to prevent breakdown of the diodes at timeswhen the high DC link voltage is discharged by power dissipating element153.

Converter Processor

[0044] Converter processor 170 can be used as the power controller andinternal control and supervision of power converter 150 for wind turbine100. In one embodiment, converter processor 170 controls the activecomponents or switches in active inverter 151 using scalar power controlwith a scalar control algorithm as described in FIGS. 4 and 5. Converterprocessor 170 can also control power dissipating element 153 using themethod described in FIG. 2.

[0045] To control these active switches using the scalar power controlwith the scalar control algorithm, converter processor 170 uses inputsignals such as generator speed f_(gen), grid voltage U_(grid), gridcurrent I_(grid), and measured rotor current values I_(rotor), for eachphase of rotor 112 (IR1, IR2, IR3). Converter processor 170 also uses agrid frequency signal indicating the operating frequency of the grid,which can be calculated from the U_(grid) signal. These input signalsand the grid frequency signal allow converter processor 170 to controlpower to the grid without performing a coordinate transformation of ACsignals. This allows for precise control of electrical quantities foreach phase of the grid because information regarding each phase of thegrid is maintained (as opposed to being lost in a transformationprocess).

[0046] Generator speed can be measured or derived, in a sensor-lesssystem, from measured electrical quantities. Generator speed is used tocontrol, among other things, the frequency of the PWM control ofinverter 151.

[0047] I_(grid) and U_(grid) indicate current and voltage measurements,respectively, on the grid. These measurements can represent current andvoltage measurements for each phase of the grid. I_(grid) and U_(grid)are also used by converter processor 170 to calculate active andreactive power and reference waveforms to control individually andindependently electrical quantities on the grid. More specifically,these signals can be used to control current and active and reactivepower for each phase of the grid as will be explained in further detailbelow.

[0048] To control power dissipating element 153 using the method of FIG.2, converter processor 170 uses a signal line connected to powerdissipating element 153. Also, converter processor 170 receives a sensedvoltage level on DC link 152 using the “DC link” signal line as shown inFIG. 1. At a normal state (below synchronous speed), the voltage levelon DC link 152 is at an acceptable threshold. At an abnormal state(above synchronous speed) caused by, e.g., a sudden wind gust, thevoltage level on DC link 152 may be above the acceptable threshold. Thisis caused by generator 110 creating excess power because of the windgust. In this situation, converter processor 170 can send a controlsignal over the connecting signal line to the lower active switch inpower dissipating element 153 such that the excess generated power isburned off or discharged in power dissipating element 153. In analternative embodiment, converter processor can control powerdissipating element 153 with time-varying signals, such as by usingpulse width modulation (PWM) signals so as to avoid overstressing powerdissipating element 153. One example of this PWM control would becontrolling the power dissipating element 153 like a brake chopper. Forinstance, the active switches in power dissipating element 153 can beselectively “turned on” or “turned off” with a selected duty cycle. Theduty cycle can be adjusted based on the DC link voltage.

[0049] The above description provides exemplary implementations ofconverter processor 170. Converter processor 170 may, alternatively oradditionally, include, e.g., separate drive circuits and controllers todrive and control the active switches in converter 151 and powerdissipating element 153. Converter processor 170 may also receive othertypes of input signals such as U_(sync). U_(sync) can represent avoltage measurement created by the magnetic buildup on stator 113 ofgenerator 110. U_(sync) can be used at start up of wind turbine 100 inthat it provides an indication of when generator 110 is to be connectedto the grid. For example, if U_(sync) is synchronized with the U_(grid)signal, generator 110 can be connected to the grid in this instance.

Power Dissipating Element Control

[0050]FIG. 2 illustrates a flow diagram of a method to control powerdissipating element 153 by converter processor 170 in FIG. 1 at belowand above synchronous speed. Initially, the process begins at stage 202.At this stage, converter processor 170 senses a voltage on DC link 152and determines if the voltage is above a threshold. For example,converter processor 170 may receive sensed DC voltage levels for DC link152 using a “DC link” input signal as shown in FIG. 1. In oneembodiment, the threshold is set above the normal voltage level or valueon DC link 152 at below synchronous speed, which may equal {square rootover (2)} times the voltage for power converter 150. For example, thethreshold may be set above 480V×{square root over (2)}. The thresholdvoltage may also be set at other voltage levels such as above 690V×{square root over (2)} if power converter 150 operates at 690V. Thethreshold voltage is preferably below a level based on the DC link 152voltage ratings to avoid damaging DC link 152. If the voltage is notabove the threshold, converter processor 170 at stage 204 maintains theactive switches in power dissipating element 153 in an off position.Because the voltage on DC link 152 is not greater than the threshold, itcan be determined that generator 110 is operating at below synchronousspeed. Thus, no measurement of generator speed is necessary to make adetermination of whether generator 110 is operating at below synchronousspeed.

[0051] On the other hand, if converter processor 170 determines thevoltage on DC link 152 is above the threshold, converter processor 170at stage 206 controls power dissipating element to turn on such that theexcess voltage from DC link 152 (or power from the rotor of generator110 at above synchronous speed) is discharged. In one embodiment, afterthis stage, converter processor 170 can turn off the power dissipatingelement 153 if it senses that the voltage on DC link 152 is at a normaloperating level such as, for example, 480V×{square root over (2)}. In analternative embodiment, converter processor 170 may turn off theswitches at a different voltage level that is acceptable for operatingturbine 100. For example, the power dissipating element 153 can bedisabled at a voltage lower than the voltage used to enable powerdissipating element 153, providing hysteresis. This threshold can alsobe adjustable or configurable based on the operating environment ofturbine 100. After the power dissipating element is turned off, theprocess may continue at stage 202 again to determine if the voltage onDC link 152 is above a threshold, or alternatively, the process may end.

Scalar Power Control and Dependent Pitch Control ProcessingConfiguration

[0052]FIG. 3 illustrates one example of a block diagram of a scalarpower control and dependent pitch control processing configuration forvariable speed wind turbine 100 consistent with the invention. Referringto FIG. 3, the basic components for wind turbine 100 include a generator110 having a rotor 112 and a stator 113. Stator 113 connects andprovides electrical power created by generator 110 to the grid. Rotor112 converts mechanical energy, which is provided by wind blades 301,into electrical energy for generator 110. Although two wind blades areshown, three wind blades, or any number of wind blades, may be used forwind turbine 100. Wind blades 301 connect to generator 110 via a mainshaft 303, gear box 302, and generator rotor shaft 111. Gear box 302connects main shaft 303 to generator rotor shaft 111 and increases therotational speed for generator rotor shaft 111.

[0053] The control processing configuration (“control system”) for windturbine 100 can be implemented in hardware as a multi-processor system.For example, although not shown, the control system may include a groundprocessor hardware unit, which is located at the bottom of the tower ofa turbine, a top processor hardware unit, which is located in thenacelle of the turbine (not shown), a hub processor turbine unit, whichis located in the hub of the turbine and rotates with turbine's blades,and a converter processor hardware unit, which is located in thenacelle. Each of these hardware units may include one or more processorchips and may be connected to each other by a suitably fast andefficient network to enable data transfer between the units, such as anAttached Resource Computer Network (ARCnet). Other interfacing protocolscould alternatively be used, such as Controller Area Network (CAN),Ethernet, FDDI, Token Ring and local area network (LAN) protocols.

[0054] Functionally, the control system may include a number ofcontrollers for controlling components within wind turbine 100 as shownin FIG. 3. Parameters such as communication speed, sample timerequirements, and processing capacity determine where portions of thefunctional blocks are physically computed (that is, which operations areperformed in which hardware unit). For example, in one implementation,the functions of the power controller are physically computed within theconverter processor. Operations for a single functional block may alsobe performed in a number of hardware units.

[0055] The control system includes a main controller 310 coupled to apower controller 312 and pitch controller 316. Main controller 310 canbe used to control the overall functions for wind turbine 100. Pitchcontroller 316 is dependent on power controller 312 through a powererror feed forward 314. Pitch controller 316 controls the pitch anglefor wind blades 301. In one embodiment, power controller 312 can controlgrid currents for each respective phase of the grid and, thereby,control active and reactive power on the grid. Power controller 312 alsocontrols power converter 150 to provide power to generator 110 and todischarge or burn off excess power from generator 110.

[0056] Main controller 310 generates and provides a main pitch referencesignal to pitch controller 316 and a power reference signal (PMG_(ref))to power controller 312. The manner in which main pitch reference signaland PMG_(ref) signal are generated will be discussed in further detailbelow. To generate the main pitch reference and PMG_(ref) signals, maincontroller 310 processes received measurements as described in moredetail in FIGS. 6 through 8. Main controller 310 may also receivecommands from a user or other internal or external processing units.Main controller 310 may also receive other types of input signals suchas, for example, temperature measurement signals indicating temperaturereadings of components or status signals on whether switches orconnections or “on” or “off” in wind turbine 100. Such input signals maybe used to control the overall operation and supervision of wind turbine100.

[0057] Power controller 312 receives PMG_(ref) signal from maincontroller 310 to determine a power error signal. The power error signalmay include information related to a calculated error for active andreactive power based on current and voltage levels for each phase of thegrid. For example, power controller 312 may calculate the power errorsignal as the magnitude of the target real power minus the magnitude ofthe measured real power. Power controller 312 also receives a generatorspeed signal from generator 110, which may be used to control componentsin power converter 150. Power controller 312 may also receive the sameinputs signals for converter processor 170 as shown in FIG. 1. Thus,power controller 312 may receive the U_(grid) I_(grid), generator speed,and current measurement I_(rotor) signals. Power controller 312 usesthese signals to control grid currents for each phase of the grid and,thereby, active and reactive power.

[0058] Power error feed forward 314 receives the power error signal frompower controller 312 and processes this signal to determine thesecondary pitch reference signal. Power error feed forward 314 allowsfor dependency between pitch controller 315 and power controller 312.The functions of power error forward feed 314 can be performed in any ofthe hardware units within wind turbine 100, e.g., the top processorhardware unit. Power error feed forward 314 allows for quick reactiontime for pitch controller 316 to respond to errors detected by powercontroller 312. That is, power error feed forward 314 ensures a quickand reliable reaction by pitch controller 316 to control the pitch forwind blades 301 so as to maintain stability for wind turbine 100.

[0059] For example, power error feed forward 314 may receive the powererror signal (i.e., the magnitude of the target real power minus themagnitude of the measured real power) from power controller 312. Basedon a nonlinear table, power error feed forward 314 generates thesecondary pitch reference signal for pitch controller 316. In otherwords, if the power error signal is considerably high, e.g., in oneembodiment higher than 20% of nominal power, this would indicate thatthe power from generator 110 is lower than expected, which means a riskof strong acceleration that may lead to an overspeed condition forgenerator 110. Power error feed forward 314 would thus set the secondarypitch reference signal to a nonzero value based on the power error frompower controller 312 and the actual pitch angle from wind blades 301 tocompensate for the error. If the power error is within tolerances, thesecondary pitch reference is set to zero.

[0060] Although described in a multi-processor system, a singleprocessor can be used to implement the functions performed by pitchcontroller 316, power controller 312, power error feed forward 314, andmain controller 310. In particular, the functions for these controllerscan be embodied in software, which can be executed by a processor toperform their respective functions.

Scalar Power Control

[0061] Wind turbine 100 uses scalar power control to control total powerand total current levels for each phase of the grid. This avoids usingcomplicated and expensive FOC processing. One purpose of scalar powercontrol is to provide a constant power output from the generator for agiven wind speed. Furthermore, scalar power control, as described below,provides more precise control of electrical quantities for each of thethree phases of the grid so as to provide optimum operation for thegrid. To implement scalar power control, wind turbine 100 uses a powercontroller 312 operating a scalar control algorithm described in FIG. 5.The following scalar power control techniques can be implemented with atime-based system. Specifically, measurements taken in real time orinstantaneously can be used to provide scalar power control.

Power Controller

[0062]FIG. 4 illustrates one example of a processing flow diagram forthe power controller 312 of converter processor 1.70. At processingstage 402, U_(grid) and I_(grid) signals are received. U_(grid) providesvoltage measurement information for each of the three phases of the gridrepresented as u_(L1)-u_(L3). I_(grid) provides current measurementinformation for each of the three phases of the grid represented asi_(L1)-i_(L3). Each voltage and current measurement for each phase isused to calculate active and reactive power, as detailed in FIG. 5. Thecalculated active power, which is represented as PMG, and the reactivepower, which is represented as QMG, are directed to processing stages403A and 403B, respectively.

[0063] At processing stages 403A and 403B, a PMG_(ref) signal and aQMG_(ref) signal are received. These signals represent ideal activepower values for a particular wind speed and derived reactive power. Atthese stages, PMG and QMG values are compared with PMG_(ref) andQMG_(ref) values. The information related to the comparison is sent topower control processing stage 405. At processing stage 405, acalculated grid frequency and generator speed information are received.This information along with information from processing stages 403A and403B are used to calculate current reference values IR1 _(ref)-IR3_(ref). These values are directed to processing stages 408A-408C,respectively. At processing stages 408A-408C, measured rotor currentsIR1-IR3 are received from rotor 112. Processing stages 408A-408Ccompares the measured current values IR1-IR3 with their respectivecurrent reference values IR1 _(ref)-IR3 _(ref). The comparisoninformation is sent to current control processing stages 410A-410C.

[0064] The current control processing stages 410A-410C determine PWMcontrol signals UR1 _(ref)-UR3 _(ref), which are sent to a PWMprocessing module 420. PWM processing module uses these signals tocontrol the active switches in active inverter 151, which then outputsnew rotor currents IR1-IR3. Because the U_(grid) and I_(grid) values foreach phase on the grid are determined by the rotor currents IR1-IR3, thepower controller 312 can control total active and reactive power and thecurrent level for each phase on the grid by controlling the rotorcurrents IR1-IR3. The control of rotor currents IR1-IR3 will bedescribed in more detail regarding the scalar control algorithm detailedin FIG. 5.

Scalar Control Algorithm

[0065]FIG. 5 illustrates a flow diagram of a method 500 for performing ascalar control algorithm by the power control 312 of FIG. 4. In oneimplementation, the scalar control algorithm is based on controllingoscillating signals. That is, the scalar control algorithm controlsoscillating rotor currents IR1-IR3 based on, e.g., a sinusoidalwaveform.

[0066] Initially, method 500 begins at stage 502, where active power PMGand reactive power QMG are calculated. This stage corresponds withprocessing stage 402 of FIG. 4. The total active power PMG can becalculated instantaneously by using the following equation:

p(t)=u ₁(t)·i ₁(t)+u ₂(t)·i ₂(t)+u ₃(t)·i ₃(t)

[0067] where u_(L1)-u_(L3) correspond to u₁(t)-u₃ (t) and i_(L1)-i_(L3)correspond to i₁(t)-i₃(t).

[0068] The total reactive power QMG can also be calculatedinstantaneously by using the following equation:${q(t)} = {\frac{1}{\sqrt{3}}\left\lbrack {{{i_{1}(t)} \cdot \left( {{u_{3}(t)} - {u_{2}(t)}} \right)} + {{i_{2}(t)} \cdot \left( {{u_{1}(t)} - {u_{2}(t)}} \right)} + {{i_{3}(t)} \cdot \left( {{u_{2}(t)} - {u_{1}(t)}} \right)}} \right\rbrack}$

[0069] where the instantaneous values for the current i(t) and u(t) canbe described as:

i(t)=î·sin(ω_(g) t+φ_(i)) and u(t)=û·sin(ω_(g) t+φ_(u))

[0070] and î is the amplitude of the current, û the amplitude of thevoltage and ω_(g) is calculated from the grid frequency f_(g). The powercalculations can be performed for each phase of the grid to obtain rotorcurrent references IR1-IR3 for each phase of the rotor.

[0071] At stage 504, a target active power (PMG_(ref)) and a targetreactive power (QMG_(ref)) are derived. The PMG_(ref) value can becalculated in main controller 310. For example, main controller 310 canuse a lookup table to determine ideal active power for a given measuredgenerator speed and rotor current. The QMG_(ref) value can be userselected. For example, the QMG_(ref) value can be selected based oneither a selectable number of variables or a selected power factor angledepending on the functions and results of reactive power compensationdesired. That is, depending on the different ways that the reactivepower is determined, a final target value QMG_(ref) is derived.

[0072] At stage 506, error signals are determined for active power andreactive power based on calculations using PMG and QMG and PMG_(ref) andQMG_(ref). For example, the PMG_(ref) is compared with PMG to generatean active power error signal and QMG_(ref) is compared with QMG togenerate a reactive power error signal. These error signals could bedetermined for each phase of the grid. This stage corresponds withprocessing stages 403A and 403B of FIG. 4.

[0073] Turbine 100 can operate as a doubly-fed turbine with rotorexcitation control (as opposed to providing reactive power and powerfactor control on the grid or line side). That is, the turbine canprovide reactive power and power factor control on the generator orrotor (or “machine” side) with a control mechanism to regulate theactive and reactive power generated on the grid by controlling rotorexcitation. At stage 508, a current reference waveform (IR_(ref)) isdetermined for the currents in the three phases of the rotor. This stagecalculates current reference waveforms (IR1 _(ref)-IR3 _(ref)). Therotor currents can be described as the sum of current components (activeand reactive), where the first part is the active component i_(rreal)responsible for the active power and the second component i_(rcomplex)is the magnetic component responsible for the reactive power such thateach instantaneous rotor current is:

i _(r)(t)=i_(rreal)(t)+i_(rcomplex)(t) and i_(r)(t)=Î _(r)·sin(ω_(r)t+β)

[0074] where the angular frequency ω_(r) for the rotor is calculated outof the rotor speed ω_(m) and the grid frequency with:

ω_(r)=ω_(g) −Ps·ω _(m)

[0075] Ps: number of pole pairs

[0076] The IR1 _(ref)-IR3 _(ref) values can be calculated in the powercontrol processing stage of FIG. 4 using measured grid frequency andgenerator speed. The calculations can be based on trigonometricfunctions, where the amplitude of the rotor-current Î_(r) is thetrigonometric sum of the active and reactive part of the desired rotorcurrent and the load angle β(∀β), which is the phase angle between thetwo components; For example, Î_(r) can be calculated using the followingequation:

Î _(r) ={square root}{square root over (i_(r real) ²+i_(r complex) ²)}

[0077] and the load angle (∀β) could be calculated using the followingequation:${\forall\beta} = {{arc}\quad {\tan \left( \frac{i_{r}\quad {complex}}{i_{r}\quad {real}} \right)}}$

[0078] At stage 510, a determination is made if each measured currentvalue or waveform matches the calculated current reference waveforms IR1_(ref)-IR3 _(ref). This stage corresponds to processing stages 408A-408Cof FIG. 4. If the waveforms match, method 500 continues back to stage510. If the waveforms do not match, an error is determined and method500 continues to stage 512.

[0079] At stage 512, electrical quantities in the rotor are adjustedsuch that each measured current waveforms (IR1-IR3) matches the currentreference waveform (IR1 _(ref-IR3) _(ref)). This stage corresponds toprocessing stages 410A-410C, and 420 of FIG. 4. In particular, based onthe determined error, desired voltage references (UR1 _(ref)-UR3 _(ref))are set for PWM processing. PWM processing uses these voltage references(UR1 _(ref)-UR3 _(ref)) to control active switches in active inverter151, which control rotor currents IR1-IR3. The above method can becontinuously performed to adjust rotor currents for each phase of therotor thereby controlling electrical quantities for each phase of thegrid.

[0080] In a similar manner, the power for each phase of the grid couldbe determined independently. In this case, the rotor currents may becontrolled such that each phase of the grid is controlled independently,making the turbine 100 responsive to asymmetry present on the grid.

Dependent Pitch Control

[0081] The main components for providing dependent pitch control aremain controller 310, power controller 312, power error feed forward 314,and pitch controller 316. The main controller 310 calculates a powerreference and a main pitch reference for the power controller 312 andpitch controller 316, respectively. The internal components of maincontroller 310 to calculate the power reference. and main pitchreference will now be explained.

[0082]FIG. 6 illustrates an internal block diagram of one implementationfor the main controller 310 of FIG. 3. Main controller 310 includes aRPM set point calculation 602 and a pitch set point calculation 604providing optimal RPM and pitch set point values. These values arechosen to allow wind turbine 100 to deliver as much electrical energy aspossible. Main controller 310 also includes a partial load controller606, switch logic 607, and full load controller 608.

[0083] The RPM set point calculation 602 receives a wind speedmeasurement to set the RPM set point value. Pitch set point calculation604 receives a measured RPM value from the generator and the wind speedmeasurement to set the pitch set point value. Partial load controller606 receives the measured RPM value, a maximum power value, and the RPMset point value to calculate the power reference (PMG_(ref)). Partialload controller 606 ensures the maximum power is not exceeded. FIG. 7describes in further detail the manner in which partial load controller606 calculates the power reference (PMG_(ref)). Full load controller 608receives the measured RPM value, pitch set point calculation value, andthe RPM set point calculation value to calculate the main pitchreference. Full load controller 608 ensures that the pitch angle is notlower than the optimal pitch angle. FIG. 8 describes in further detailthe manner in which full load controller calculates the main pitchreference.

[0084] Referring to FIG. 6, switch logic 607 provides an enable signalto both partial load controller 606 and full load controller 608. Theenable signal controls when portions of the partial load controller 606and full load controller 608 are enabled to operate as will be describedbelow in FIGS. 7 and 8.

[0085]FIG. 7 illustrates an internal block diagram of one implementationfor the partial load controller 606 of FIG. 6. In one embodiment,partial load controller 606 is active only when the turbine power isoperating at less than maximum power output. Referring to FIG. 7, acomparator 701 compares the measured RPM value with RPM set pointcalculation to determine an RPM error (e.g., RPM set point—measuredRPM). This error is sent to PI controller 704 via gain scheduling 702,which also receives the RPM set point signal. Gain scheduling 702 allowsthe amplification (gain) for partial load controller 605 to be dependenton a certain signal, i.e., the RPM set point signal. PI controller 704generates the power reference signal using the error signal from gainscheduling 702. In one embodiment, if the power reference signal exceedsthe maximum power, a signal is sent to switch logic 607 to cause switchlogic 607 to disable partial load controller 606 and enable full loadcontroller 608, and the output will be clamped by controller 606 to themaximum power.

[0086]FIG. 8 illustrates an internal block diagram of one implementationfor the full load controller 608 of FIG. 6. In one embodiment, full loadcontroller 608 is active only when the wind turbine power is equal tothe maximum power. If the wind speed is high enough, it may produce toomuch power and the turbine components may become overloaded. In thissituation, the RPM generator speed is also controlled by moving thepitch angle away from the maximum power position for the wind blades.

[0087] Referring to FIG. 8, a comparator 801 compares the RPM set pointwith measured RPM to determine an RPM error (e.g., RPM setpoint—measured RPM). This error is sent to PI controller 805 via gainscheduling I 802 and gain scheduling II 804. Gain scheduling I 802receives the RPM error and gain scheduling II 804 receives main pitchreference signal. Gain scheduling I 802 and II 804 control gain for fullload controller 608 dependent on RPM error and main pitch reference. PIcontroller 805 generates the main pitch reference signal using the RPMerror. In one embodiment, if the main pitch reference is lower than themaximum power set point, a signal is sent to switch logic 607 to causeswitch logic 607 to disable full load controller 608 and enable partialload controller 606, and the output will be clamped to the maximum powerproducing pitch set point. Main controller 310, however, can use othermore complicated pitch and power reference generating schemes thatensure reduction of loads, noise, etc. For example, partial loadcontroller 606 and full load controller 608 could use the power errorfeed forward signal to quickly react to a large power error.

[0088]FIG. 9 illustrates a block diagram of one implementation for thepitch controller 316 of FIG. 3. Referring to FIG. 9, pitch controller316 includes a comparator 906 that compares a secondary pitch referencesignal from power error feed forward 314, main pitch reference signalfrom main controller 310, and a measured pitch angle from pitch system910 to determine a pitch error. The pitch error can be, e.g.,[(mainpitch reference+secondary pitch reference)—measured pitch angle]. Anon-linear P-controller 908 provides a control voltage to a pitch system910 based on the pitch error. Pitch system 910 connects with one of thewind blades 301 and includes components to control the pitch of the windblade. For example, pitch system 910 may include a hydraulic systemwhere the control voltage is applied to a proportional value thatgenerates a hydraulic flow moving a pitch cylinder that controls thepitch of a wind blade. The pitch position can be monitored by thedisplacement of the cylinder and feedback to comparator 906. The samplerate for pitch controller 316 can be set at a low value compared to thesample rate for power controller 312. For example, pitch controller 316could operate at 50 Hz while power controller 312 could operate at 5Khz.

[0089] Thus, a variable speed wind turbine is provided having a passivegrid side rectifier with scalar power control and a pitch controlleroperating dependently with a power controller. Furthermore, while therehas been illustrated and described what are at present considered to beexemplary implementations and methods of the present invention, variouschanges and modifications may be made, and equivalents may besubstituted for elements thereof, without departing from the true scopeof the invention. In particular, modifications may be made to adapt aparticular element, technique, or implementation to the teachings of thepresent invention without departing from the spirit of the invention.

[0090] In addition, while the described implementations include hardwareembodiments, which may run software to perform the methods describedherein, the invention may also be implemented in hardware or softwarealone. Accordingly, the software can be embodied in a machine-readablemedium such as, for example, a random access memory (RAM), read-onlymemory (ROM), compact disc (CD) memory, non-volatile flash memory, fixeddisk, and other like memory devices. Furthermore, the processors andcontrollers described herein can execute the software to perform themethods described above. Other embodiments of the invention will beapparent from consideration of the specification and practice of theinvention disclosed herein. Therefore, it is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1. (Canceled)
 2. (Canceled)
 3. (Canceled)
 4. (Canceled)
 5. (Canceled) 6.(Canceled)
 7. (Canceled)
 8. (Canceled)
 9. (Canceled)
 10. (Canceled) 11.(Canceled)
 12. (Canceled)
 13. (Canceled)
 14. A method of operating avariable speed wind turbine, comprising: configuring at least two phasesof a stator or a generator for connection to a utility grid; providing aplurality of switches on the rotor-side of a generator; and controllingthe switches such that electrical quantities for at least two phases ofthe utility grid are controlled independently.
 15. (Canceled) 16.(Canceled)
 17. (Canceled)
 18. (Canceled)
 19. (Canceled)
 20. (Canceled)21. A method for controlling power from an electrical generator to apower grid comprising: receiving voltage values and current values ofthe power grid relative to a time-based system; calculating active andreactive power from the voltage and current values without convertingfrom the time-based system; determining a power error based on theactive and reactive power; generating current reference values tocontrol rotor currents in the electrical generator based on thedetermined power error without converting from the time-based system;and controlling the rotor currents in the electrical generator based onthe current reference values.
 22. The method of claim 21, wherein thereceiving of the voltage values and current values includes receivingthe voltage and current values for each phase of the power grid.
 23. Themethod of claim 22, wherein the calculating of the active and reactivepower includes calculating the active and reactive power for each phaseof the power grid using the voltage and current values for each of thepower grid.
 24. The method of claim 21, wherein the generating of thecurrent reference values includes generating the current referencevalues based on the determined power error, a power grid frequency, anda generator speed.
 25. The method of claim 21, wherein the controllingof the rotor currents in the electrical generator includes controllingthe rotor currents for each phase of the rotor in the electricalgenerator based on the current reference values.
 26. The method of claim21, further comprising: controlling electrical quantities in each phaseof the power grid by controlling the rotor currents in the electricalgenerator.
 27. A system for controlling power from an electricalgenerator to a power grid comprising: means for receiving voltage valuesand current values of the power grid relative to a time-based system;means for calculating active and reactive power from the voltage andcurrent values without converting from the time-based system; means fordetermining a power error based on the active and reactive power; meansfor generating current reference values to control rotor currents in theelectrical generator based on the determined power error withoutconverting from the time-based system; and means for controlling therotor currents in the electrical generator based on the currentreference values.
 28. The system of claim 27, wherein the means forgenerating current reference values includes means for generatingcurrent reference values without converting from a time-based system.29. The system of claim 27, wherein the means for receiving of thevoltage values and current values includes means for receiving thevoltage and current values for each phase of the power grid.
 30. Thesystem of claim 29, wherein the means for calculating of the active andreactive power includes means for calculating the active and reactivepower for each phase of the power grid using the voltage and currentvalues for each of the power grid.
 31. The system of claim 27, whereinthe means for generating of the current reference values includes meansfor generating the current reference values based on the determinedpower error, a power grid frequency, and a generator speed.
 32. Thesystem of claim 27, wherein the means for controlling of the rotorcurrents in the electrical generator includes means for controlling therotor currents for each phase of the rotor in the electrical generatorbased on the current reference values.
 33. The system of claim 27,further comprising: means for controlling electrical quantities in eachphase of the power grid by controlling the rotor currents in theelectrical generator.
 34. (Canceled)
 35. (Canceled)
 36. (Canceled) 37.(Canceled)
 38. (Canceled)
 39. (Canceled)
 40. A method for a variablespeed wind turbine comprising: generating rotor currents for anelectrical generator connected to wind blades, the rotor currentscorresponding to a plurality of phases of a power grid; and controllingindependently the rotor currents for each phase of the power grid. 41.The method of claim 40, wherein controlling independently the rotorcurrents includes controlling independently rotor currents for eachphase of the power grid based on electrical characteristics of eachphase of the power grid.
 42. A variable speed wind turbine comprising: agenerator connected to wind blades that provides rotor currents, therotor currents corresponding to a plurality of phases of a power grid;and a controller to independently control the rotor currents for eachphase of the power grid.
 43. The variable speed wind turbine of claim42, wherein the controller controls independently rotor currents foreach phase of the power grid based on electrical characteristics of eachphase of the power grid.
 44. A method for a variable speed wind turbinecomprising: generating electrical power for a power grid using a rotorand stator of an electrical generator; and controlling active switchesin a power converter using a scalar control algorithm that controls eachphase of the rotor based on measured electrical quantities of each phaseof the power grid.
 45. The method of claim 44, wherein controlling theactive switches includes controlling the active switches using pulsewidth modulation (PWM) techniques.
 46. The method of claim 44, furthercomprising: receiving at least power grid frequency values and rotorcurrent values for each phase of the rotor to control the activeswitches.
 47. The method of claim 44, further comprising: controllingthe electrical power to the power grid without performing a coordinatetransformation of AC signals.
 48. A variable speed wind turbinecomprising: an electrical generator having a rotor and stator togenerate electrical power for a power grid; and a power converter tocontrol the generated electrical power, the power converter includingactive switches controlled by a scalar control algorithm that controlseach phase of the rotor based on measured electrical quantities of eachphase of the power grid.
 49. The variable speed wind turbine of claim48, further comprising a controller to control the active switches ofthe power converter.
 50. The variable speed wind turbine of claim 49,wherein the controller controls the active switches using pulse widthmodulation (PWM) techniques.
 51. The variable speed wind turbine ofclaim 49, wherein the controller receives at least power grid frequencyvalues and rotor current values for each phase of the rotor to controlthe active switches.
 52. The variable speed wind turbine of claim 51,wherein the controller controls electrical power to the power gridwithout performing a coordinate transformation of AC signals.
 53. Amethod for a power controller of a variable speed wind turbinecomprising: receiving power grid related information for a power grid;calculating active and reactive power values for the power grid based onthe received power grid related information; comparing the calculatedactive and reactive power values with reference active and reactivepower values, respectively; generating current reference values based onthe comparison of the calculated active and reactive power values withthe reference active and reactive power values; comparing the generatedcurrent reference values with measured current values; and generatingnew current values to control the active and reactive power for thepower grid based on the comparison of the generated current referencevalues with the measured current values.
 54. The method of claim 53,wherein the power grid related information includes voltage and currentmeasurement values for each phase of the power grid.
 55. The method ofclaim 53, further comprising: generating control signals to controlactive switches that output the new current values.
 56. The method ofclaim 55, wherein the active switches are controlled using pulse widthmodulation (PWM) techniques.
 57. A computer-readable medium containinginstructions, which if executed by a computing system, causes thecomputing system to perform a method comprising: receiving voltagevalues and current values of a power grid relative to a time-basedsystem; calculating active and reactive power from the voltage andcurrent values without converting from the time-based system;determining a power error based on the active and reactive power;generating current reference values to control rotor currents in anelectrical generator based on the determined power error withoutconverting from the time-based system; and controlling the rotorcurrents in the electrical generator based on the current referencevalues.
 58. A computer-readable medium containing instructions, which ifexecuted by a computing system, causes the computing system to perform amethod comprising: receiving power grid related information for thepower grid; calculating active and reactive power values for the powergrid based on the received power grid related information; comparing thecalculated active and reactive power values with reference active andreactive power values, respectively; generating current reference valuesbased on the comparison of the calculated active and reactive powervalues with the reference active and reactive power values; comparingthe generated current reference values with measured current values; andgenerating new current values to control the active and reactive powerfor the power grid based on the comparison of the generated currentreference values with the measured current values.